To check the transit time of mail items, it is known to use so-called transit time testers which are posted with the mail item to be transported and contain devices by means of which the sequence of movement of the mail items is registered. Known devices of this type contain motion sensors which register the movement of the consignment over its entire mailing time. Forces occurring during the transportation are registered in a motion/time diagram; if the consignment is at rest, i.e. there is no transport, there is no recording, either. The recorded motion/time diagram can be evaluated later at a central position. A nominal/actual comparison enables any stops in the delivery or mailing to be localized since the routes of transportation and transportation times are known as a rule.
Transit time monitoring devices in conventional letter thickness are also known which have a memory for detecting the measurement values and evaluating electronics. These transit time monitoring devices are arranged in such a manner that they can be processed in mail sorting machines and are not separated during the stiffness measurement in the mail processing machines of the post offices. Such devices can be used, e.g. for determining whether the consignment has been completely at rest in an inadmissible manner for a number of days after transportation for several hours.
The known devices have the disadvantage that they only provide for detection of the states of motion and of rest but do not allow any more accurate distinction of the type of movement actually occurring during a motion state, or even of the means of transportation used.
A device for monitoring the transit time of goods to be conveyed, by means of which the means of transportation, transport events and types of movement used during the mailing time can be identified, is known from DE 44 04 195 C1. It consists of an acceleration sensor, a microcontroller with integrated analog/digital converter and a RAM memory. The device is arranged in the format of a standard C6 letter in such a manner that it can be processed in mail sorting machines and is not separated out during the stiffness measurement in the letter processing machines in the post offices. In all process steps of letter conveyance and processing, the device behaves like a normal letter of paper. This ensures the reliability of the data obtained in the letter conveyance and processing process and an increase in the functional reliability and usage period.
A motion sensor in a “letter” constructed in this manner outputs a sensor signal proportional to acceleration which is digitized by the analog/digital converter (ADC). The signal is processed further in a microcontroller to form frequency spectra which are stored in compressed form in the memory. After the conclusion of the recording of the measurement values, the stored spectra are read out and evaluated. During this process, the frequency spectra are correlated in time with the temporal sequence of the movement of the device during the transportation. Since the various transport media such as, for example, motor vehicles, rail, transportation on foot or flight in each case display characteristic spectral variations, the means of conveying can be identified, in the most favorable case, by means of the variation with time of the transportation process.
In a further known method, so-called quality test letters (QTL) are also used in which the physical characteristics during the transportation are recorded with time and then read out and classified in accordance with the steps of the transportation process. In this context, all possible nominal transport sequences of each QTL consignment sent out with the selected conditions of conveyance are automatically generated from defined transport rules between the nodes of the logistic network and from the description of the sequences in the nodes and the relations of the nodes with one another. The QTL consignments are identified in the nodes and the actual transport sequence is determined from the location/time relations. Weak points can be determined by comparing the nominal transport sequences and the actual transport sequence.
However, the known devices cannot be used for obtaining precise location information. For the determination of the current location, only two methods are essentially known: the radio cell location and position finding with the aid of satellite systems (e.g. GPS, Galileo or GLONASS).
The accuracy of radio cell location depends on the size of the radio cells. As a result, the accuracy of the position determination also differs greatly. Depending on the prevailing conditions, it is between 50 m and up to 30 km and more. For this reason, radio cell location is not suitable for the purposes of transit time monitoring and position determination of mail items.
Much more accurate position determination is possible with the aid of satellite systems. Known GPS data loggers (GPS loggers in the sense of the patent are all devices which can determine and store the current geographic position of the device from satellite data, independently of the satellite system used) exclusively determine position information as a function of time. The current generation of processors for GPS data loggers with internal signal processing for calculating the current geographical coordinates consume much energy, however, which only plays a subordinate role in monitoring means of conveyance and relatively large transportation units (such as, e.g. containers and vehicles). However, the energy demand is so high that it is not possible to implement a GPS data logger for a transit time of a number of days which corresponds in size, thickness, weight and stiffness to a standard letter consigned, and, at the same time, withstands the enormous loads in mail sorting machines.
A further disadvantage is that GPS receivers always need a relatively free view of the satellite. If the mail consignments are conveyed, for example, in a steel container or if the mail consignment is in a metal mailbox, the satellite signal cannot be received. Once the GPS receiver can again receive the satellite signals, it needs a relatively long time for being able to determine its position again since the current position of the GPS receiver is unknown. As a result, very much energy is used over a relatively long time. However, GPS receivers are known which detect rest by means of acceleration sensors and increase the cycle time for determining the geographical coordinates in the case of relatively long rest phases. This makes it possible to save energy.